The simplest mechanical automatic transfer switch comprises a single form C relay. It is desirable to have the relay switch quickly to minimize the power disruption to the load. However, rapid relay switching creates a possible shoot-through problem, i.e., an arc will form between the opening contacts, and if this arc is still conducting when the closing contacts close, a current path is created between the two input sources, shorting them together through the arc.
This problem was addressed in, for example, U.S. Pat. No. 4,811,163 which discloses the use of solid state snubbers in parallel with two mechanical form A or form B relays, with the relays used to accomplish the transfer. The arc is quenched by the solid state snubbers in parallel with the opening switch contact. Conversely, the present invention uses a relay in series with the contact for arc quenching rather than a parallel snubber. Furthermore the control circuit timing used in the present invention has significant advantages over the control circuit disclosed in the '163 Patent. The control circuit of the '163 Patent uses a pair of solid state switches to short the inputs together if one fails or is fired by noise or a failure of the drive circuit. This creates a potential backfeed problem, i.e., if one of the inputs is unplugged, a person touching the power pins on the end of the cord could be shocked by power coming from the other source. Furthermore, if both circuits are powered, a catastrophic short circuit would result. These safety problems render the system disclosed in the '163 Patent unacceptable under UL safety standard UL1950. Conversely, the system of the present invention meets these safety requirements.
An additional challenge facing designers and users of automatic transfer switches is detecting a failure of the primary source, so that a transfer to the secondary source can be initiated. A typical technique is to extract the level of the AC signal as a DC signal and compare it to a fixed DC reference. Most of the known techniques for detecting the level of an AC signal by converting it to a DC signal require long time constant filters to remove the AC component of the signal. Digital voltmeters, for example, use either a rectifier or an RMS to DC converter followed by a long time constant, low pass filter to smooth the ripple. These long time constant filters have long delays that are unacceptable in a transfer switch application, which must detect an AC signal failure in a quarter cycle or less.
One known technique to avoid this problem is to use a computer chip to compare the voltage in real time to an ideal sine wave reference signal calculated by the CPU. A transfer is initiated if the voltage deviates from the ideal sine wave by more than a predetermined amount. One shortcoming of this technique is that a dead band exists around the zero crossings of the voltage waveform. Because the source voltage is nearly zero during this portion of the cycle, it is difficult to differentiate between the normal waveform zero crossing and a source failure. One prior art solution to this problem has been to wait a sufficient time until it is known that the voltage is supposed to be higher. If he voltage has not risen, a failure has occurred. In addition to the undesirable delay, an additional disadvantage of this technique is that it requires a CPU, with the associated complexity, noise and reliability problems.
Conversely, the automatic transfer switch of the present invention solves this problem by tracking the slope of the AC signal in addition to its magnitude. Because the slope of a sine wave is highest at the zero crossings, the slope signal is strongest at exactly the same point where the magnitude signal is weakest. Therefore, adding the magnitude and slope creates a signal that reliably and quickly indicates a voltage source failure at all points along the waveform.